1. Field of the Invention
The present invention relates to a method of manufacturing a sensor having a membrane structure. The sensor is used, for example, for measuring an amount of air supplied to an internal combustion engine.
2. Description of Related Art
A sensor having a membrane structure that includes metal stripes is known hitherto. Such sensor measures a physical amount based on physical property changes of the metal stripes. For example, a heat-sensitive airflow sensor having metal stripes made of platinum or the like measures an amount of air supplied to an internal combustion engine. An amount of heat radiated by an airflow is detected by the metal stripes, and the amount of air is measured based on the detected amount of radiated heat.
An example of a conventional airflow sensor having such a membrane structure will be briefly explained in reference to FIGS. 7A-7C. The membrane structure of the conventional airflow sensor is formed in a process shown in FIGS. 7A-7C. As shown in FIG. 7A, a first insulating layer 101 is formed on a substrate (not shown), and then a metal layer 102, e.g., 0.2 xcexcm thick, is formed on the first insulating layer 101. Then, a resist layer 103 having about 1 xcexcm thickness is formed on the metal layer 102. Then, as shown in FIG. 7B, the metal layer 102 is patterned into metal stripes 104 having a desired form in an etching process such as ion-milling, using the resist layer 103 as a mask. Then, a heat-treatment such as annealing is performed to improve properties of the metal stripes 104. By the heat-treatment, a temperature coefficient of resistance (TCR) of the metal stripes 104 is raised thereby to improve a temperature sensitivity of the metal stripes 104.
Then, as shown in FIG. 7C, a second insulating layer 105 is formed to cover the metal stripes 104. Then, a cavity is formed from a bottom of the substrate at a position corresponding to the metal stripes 104 and its vicinity. Thus, a membrane structure having the insulation layers 101, 105 and the metal stripes 104 is formed.
Since a taper angle xcex81 (shown in FIG. 7A) formed between the upper surface of the metal layer 102 and an end slope of the resist layer 103 is large (about 80 degrees), a taper angle xcex82 (shown in FIGS. 7B and 7C) formed between the upper surface of the first insulating layer 101 and an end slope of the metal stripe 104 also becomes large, because the taper angle xcex82 is influenced by the taper angle xcex81 in the etching process. As a result, as shown in FIG. 7C, cracks 105a are developed at stepped corners of the second insulating layer 105 in the process of forming the second insulating layer 105. The mechanical strength of the membrane structure is reduced by the cracks 105a. Therefore, there is a possibility that the membrane structure is broken down by a heat stress generated by on-and-off operation of the airflow sensor, because a thermal expansion coefficient of the insulating layers 101, 105 is different from that of the metal stripes 104.
To prevent development of the cracks 105a, it would be effective to reduce the taper angle xcex81 thereby to reduce the taper angle xcex82. For this purpose, a process shown in FIGS. 8A and 8B is conceived. In the process shown in FIG. 8A, a thinner resist layer 106 is coated and the resist layer is smoothened by performing post-baking, thereby making the taper angle xcex81 smaller. Using the resist layer 106 having a smaller taper angle xcex81 as a mask, the metal layer 102 is etched into a form of the metal stripes 104. In this manner, the metal stripes 104 having smaller taper angle xcex82 can be formed as shown in FIG. 8B. It may be also possible to make the taper angle xcex82 smaller by supplying etching gas in a direction slanted from a line perpendicular to the upper surface of the metal layer 102, even a resist layer having a large taper angle xcex81 is used as a mask.
However, even if the metal stripes 104 having a small taper angle xcex82 is formed by etching, the taper angle xcex82 is increased by heat-treatment performed after the etching process to improve the properties of the metal stripes 104. This is because a grain size in the metal stripes 104 is increased in the heat-treatment process. Because the grain size is also enlarged at a tip of the tapered portion, the taper angle xcex82 is increased accordingly. As a result, cracks 105a develop due to the large taper angle xcex82. To confirm the crack development, tests were carried out. As test samples, the metal stripes having various taper angles xcex82 in a range 32-72 degrees were made. Such metal stripes were heat-treated after they were formed, and whether the cracks developed or not were observed. The cracks 105a were found in all the samples.
Further, there is a possibility that electric-filed concentrates at end portions of the metal stripes 104 in the etching process of the metal layer 102, and thereby the first insulating layer 101 is over-etched, forming steps 101a as shown in FIG. 9. When the second insulating layer 105 is formed on the over-etched steps 101a, there is a possibility that the cracks 105a develop at the steps 101a as shown in FIG. 9.
In a conventional membrane structure in which the metal layer 102 is composed of tow layers, there is a following problem. FIGS. 16A-16D briefly show a manufacturing process of such a membrane structure. As shown in FIG. 16A, a first insulating layer 101 and a metal layer 102 composed of a titanium (Ti) contact layer 102a and a platinum (Pt) layer 102b are formed on a substrate (not shown). Then, as shown in FIG. 16B, the metal layer 102 is annealed and a resist layer 103 is formed on the metal layer 102. In the annealing process, titanium grains 102a are dispersed into the platinum layer 102b. 
Then, as shown in FIG. 16C, the metal layer 102 is etched by ion-milling, using the resist layer 103 as a mask, to form metal stripes 104. In the etching process, the first insulating layer 101 is over-etched, thereby forming steps 101a and forming a large taper angle xcex82 at the tip of the metal stripe 104. Further, titanium grains 102a are left over on the tapered surface because a milling speed of titanium is slower than that of platinum. Also, residual spots 101b are formed on the surface of the first insulating layer 101 because the shape of the titanium grains 102a is projected on the surface where the titanium grains exist in the platinum layer 102b. 
Then, a second insulating layer 105 is formed to cover the metal stripes 104, as shown in FIG. 16D. Cracks 105a and voids 105b develop in the second insulating layer 105 due to presence of the titanium grains 102a on the tapered surface and the steps 101a on the surface of the first insulating layer 101. Further, due to the residual spots 101b, projections are formed on the surface of the second insulating layer 105. The mechanical strength of the membrane structure is considerably reduced by the cracks 105a, voids 105b and projections.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a manufacturing method of a sensor device, in which a mechanical strength of a membrane structure is improved by preventing development of cracks in an insulating layer.
A membrane structure composed of thin film layers is formed on a part of a substrate such as a silicon substrate. The membrane structure includes a first insulating layer, a second insulating layer and metal stripes formed between both insulating layers. The metal stripes are formed by etching a metal layer disposed between both insulating layers. Electrical resistance of the metal stripes changes according to temperature, and the resistance changes are used to detect physical amounts such as an amount of air flowing through the metal stripes.
The membrane structure is manufactured as follows. The first insulating layer made of a material such as silicon oxide is formed on the substrate. The metal layer made of a material such as platinum is formed on the first insulating layer. The metal layer is heat-treated to enlarge its grain size and to increase its temperature coefficient of resistance. The heat-treated metal layer is etched into a form of the metal stripes using a resist layer formed on the metal layer as an etching mask. Then, the second insulating layer made of a material such as silicon oxide is formed to cover the metal stripes.
Since the heat-treatment of the metal layer is performed before the etching process, an end-portion taper angle of the metal stripes is not increased by the heat-treatment. Therefore, the second insulating layer can be formed on the metal stripes without generating cracks therein.
In the process of etching the metal layer, a thin resist layer is formed on the metal layer, and the resist layer is post-baked at a relatively high temperature to flatten its surface and thereby to decrease its end-potion taper angle. Using the resist layer as an etching mask, the metal layer is etched so that the taper angle of the metal stripes becomes smaller than 65 degrees. The etching may be performed under an ion-milling process using argon gas. In order to prevent the first insulating layer from being over-etched in the process of etching the metal layer, an etching-stopper layer made of a material such as titanium may be additionally formed underneath the metal layer to cover the first insulating layer.
The metal layer in the membrane structure may be composed of a double layer that includes a first metal layer made of a metal such as titanium and a second metal layer made of a metal such as platinum. The first metal layer functions as a contact layer disposed between the first insulating layer and the second metal layer to improve their close contact. The metal layer composed of the double-layer is etched so that both metals, e.g., platinum and titanium, are etched at a substantially same etching rate in order to obtain a smooth tapered end-surface of the metal stripes.
The metal layer composed of the double layer may be etched under an ion-milling process, in which an ion-incident angle upon the metal layer is set to an angle slanted from a normal line of the metal layer surface, and the metal layer is rotated around the normal line during the etching process. The ion-incident angle is preferably set to 20-60 degrees to effectively suppress formation of etched steps on the first insulating layer. In this manner, the metal stripes having smooth tapered end surfaces are formed, and the steps on the first insulating layer are suppressed. Therefore, the second insulating layer is formed on the metal stripes without generating cracks in the second insulating layer.
It is also possible to perform an additional etching for removing residual spots formed on the surface of the first insulating layer. By additionally etching the surface of the first insulating layer in an amount of 30% of the metal layer thickness, the residual spots on the first insulating layer is effectively removed. Thus, the second insulating layer is formed on the smoothened surface of the first insulating layer, and thereby formation of the cracks in the second insulating layer is avoided.
According to the present invention, formation of the cracks in the second insulating layer is prevented, and thereby the membrane structure having a sufficient mechanical strength is manufactured.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.